A typical Orbal oxidation ditch (shortened to “Orbal” in English) comprises an anoxic channel, a facultative anaerobic channel, an aerobic channel and a central island, outer channel, median channel and inner channel for short respectively. The tank bodies generally assume the shape of an ellipse with some of them in the shape of round and the deposition tank is set in the center.
As disclosed in the Chinese Patent No. CN03223656.5 that “A wastewater treatment device using A2/O oxidation ditch process”, the device consists of an anaerobic tank, an anoxic tank, an oxidation ditch and a deposition tank, wherein the combination of the deposition tank and the oxidation ditch forms the tank body in the shape of concentric double-walled cylinder with the oxidation ditch around the outer side, the deposition tank inside and the anoxic tank located outside of the oxidation ditch as well as very near to the anaerobic tank outside of the anoxic tank.
Although this design saves the space between tanks, and the connecting pipes in shared walls between tank bodies, the round deposition tank at the central island is an individual one and the whole oxidation ditch body has a huge volume without denitrification system, whereas a high-denitrification internal cyclic system is needed for the Orbal oxidation ditch having total nitrogen removal requirements.
In accordance with the code for the design of water supply and sewerage in China's cities, the residence time in oxidation ditch shall be at least 16 hours. The contradiction between the residence time and the volume of the oxidation ditch, the establishment of individual denitrification system limit the popularization of this kind of wastewater treatment device. While other treatment systems in need of anaerobic system and deposition tank further solely-built also lead to the fact that the Orbal oxidation ditch is incapable of being widely applied. Especially, the outer channel of the Orbal oxidation ditch is over wide (based on process requirements, the outer channel covers more than 50% of the total oxidation ditch in volume, which results in the reduction of media circulation times, and particularly, too narrow spacing between the aeration turntables set on the outer channel creates unobvious diversities between aerobic-anaerobic-aerobic . . . , unobvious nitrification and denitrification effects, and unobvious denitrification effect apparently, thus removing the total nitrogen only a little.